It all depends on your goals. If you are only doing this for kicks and grins and seriously don't plan to compete, any car will be fine. You can learn performance driving skills in all of them.
If you would like to compete at some point, choose a car with a large class (Boxster/944) and build it to be class competitive. Then you will have some great benchmarks to measure your driving skill against. The last thing you want to do is buy/build a one-off frankenPorsche that gets stuffed in Unlimited class against the 800hp shop-owner specials running a fresh set of Yoko slicks every event. If you want to compete, compete to win. Winning can be fun. No one wants to hold down DFL every event because you stuffed yourself in with the bottomless budget Disney and Walton boys.
Seriously, if you want to start doing DEs, pick a car and go. Just have decent tires, good brakes and make sure it is mechanically sound. Lots of opportunities in SoCal and a stock car is often the best place to start. After you have a dozen track days experience the mod choices will become much more clear. Buy some fresh tires and get out there!
Here are some upcoming DEs near you that accept novice drivers in stock cars:
http://www.zone8.org/calendar.php
This might be a great place to begin your journey into performance driving. It is arguably one of the best schools in the country:
http://www.pcasdr.org/event/pca-sdr-performance-driving-school-friday-evening-chalk-talk-black-forest/